According to the development of vapor-phase synthesis of diamond, development of diamond coating tools formed by vapor-phase synthesis has been actively pursued, wherein diamond material has been coated on a base material for tools such as cutting chips, drills, and end mills. In the case of directly coating the diamond on a base material for a tool by direct vapor-phase synthesis, the diamond coating may peel off from the base material depending on the method due to the stress resulting from thermal expansion coefficient difference between the materials of the base material and diamond, that is to say, residual stress produced from temperature difference during coating of the diamond on the base material, during cooling after coating, and therefore the productis deficient for practical usage. On the other hand, diamond material has been coated in advance on a specific base material by vapor-phase synthesis, and then the base material covered with the diamond coating obtained has been soldered directly attaching the base material and tool; or the base material has been removed by polishing after soldering, directly attaching the diamond surface and tool; or the diamond coating alone has been soldered to the tool after peeling the diamond coating from the diamond coated base material. In these methods, the thicker the diamond coating is, the more the residual stress increases, and in case of a thin base material, warpage occurs of the base material covered with the diamond coating. Once warpage occurs, troubles increase such as a longer polishing time is required to polish the coating surface, or the diamond coating is easily cracked during soldering to the tool. For example, laid-open Japanese patents Hei 1-167211, Hei 3-197677, and Hei 4-992278 disclose that formation of the coating is carried out in plural steps in order to improve tenacious adherence during forming of the diamond coating on the base material, and so the precipitated coating has a multilayered structure. Moreover, laid-open Japanese patent Hei-306195 proposes prevention of warpage of the coating during forming the diamond coating.
Laid-open Japanese patent Hei 1-167211 discloses a diamond-like carbon coating and the synthesis thereof having small coating stress and smooth surface flatness comprising diamond like carbon layer including a high content of amorphous carbon and that include a low content of amorphous carbon. The objective of this patent is to obtain a diamond like carbon layer having a flat surface and high light transmissivity for coating of a lens or window and providing a protective film for a magnetic disk, and it produces a coating of diamond like carbon formed by alternate precipitation of diamond like carbon layer including a high content of amorphous carbon and that includes a low content of amorphous carbon was obtained by periodically changing the temperature of the base material, electromagnetic wave energy such as microwave and high frequency, concentration of carbon source, and reaction pressure. However, the objective of this patent is to obtain a diamond like carbon layer having flat surface and high light transmissivity, and in order to obtain a coating having smooth surface flatness without surface polishing such as lapping after forming coating, the uppermost surface layer needs to have smooth surface flatness but to have low hardness of diamond like carbon layer including high content of amorphous carbon; and therefore it is not suitable for use as a hard coating layer for a tool due to insufficient wear resistance, which is the objective of the present invention.
Laid-open Japanese patent Hei 3-197677 is discloses a diamond coated tool and the manufacturing process thereof which has multilayers of diamond layer coated on the base material such as cemented carbide, cermet, silicon nitride sintering, or aluminum oxide sintering, in which the impurity content of the first layer is higher than that of the second layer and in which grain size of diamond of the first layer is smaller than that of the second layer. This patent needs different processes for formation, respectively, in which the coating of the first layer is formed by filament CVD, and then the coating of the second layer is formed by plasma CVD. Therefore, it has some problems, for example, slow productivity and the possibility of forming defects in the second layer due to deposition of impurity such as particles of dust on the coating surface after forming the first layer.
Laid-open Japanese patent Hei 4-99278 discloses a diamond coated tool having multilayered diamond coating layer having a thickness of 0.5-200 .mu.m formed on a hard material, which has not less than one layer of the diamond coating icluding a carbon component except for diamond, and not less than one layer without it, respectively. This patent intends to relax the impact of the whole coating layer received and to have peeling resistance and wear resistance at the same time by forming a high toughness diamond layer including a carbon component as amorphous carbon type which is the same component as diamond between (1) the diamond layer without carbon component except for diamond and (2) the base material. However, stress formed into the coating increases as the thickness of the coating increases; therefore, this system has a problem that the coating is easy to peel off.
Laid-open Japanese patent Hei 5-306195 discloses a manufacturing process of CVD diamond thin film having internal tensile stress released and flatness, because the forming surface of the base material for the precipitated diamond coating is a convex shape having a curvature diameter which compensates for internal tensile stress foreseeable in the diamond thin film obtained by removal from the base material, whereby the thin film removed from the base material warps in an opposite direction of the curvature of base material. A characteristic of this patent is to foresee the degree of warpage of the coating formed by internal stress in the coating during precipitation of the diamond coating on the base material beforehand and to carry out the coating of the base material as a convex shape for compensating warpage of the coating beforehand in order to be a flat layer after releasing internal stress by removing the coating from the base material. However, it is lacking in practical use, since the forming condition of the diamond coating should precisely be controlled due to the shape of the convex base material for the reason that internal stresses created in the coating vary depending on the forming equipment and the forming conditions, for example, the greater the forming velocity of a coating, the greater the internal stress which is created in the coating.